1. Field of the Invention
The present invention relates to a demodulation method and a demodulator that demodulates signals, which have been modulated based on the Frequency Shift Keying (FSK) modulation scheme.
2. Description of Related Art
A FSK modulation is known as a conventional modulation scheme used for sending digital signals from wireless transmitters. A FSK modulator, which performs the FSK modulation, modulates a frequency signal of a carrier wave so that different carrier frequencies represent signal levels of the digital signals.
FIG. 15 is a block diagram showing a typical arrangement of a FSK receiver 102 that has a demodulator for demodulating FSK-modulated signals. The FSK receiver 102 includes an antenna 104, a radio frequency band-pass filter (RF-BPF) 106, an RF amplifier 108, a local oscillator 110, a mixer 112, an intermediate frequency band-pass filter (IF-BPF) 114, an IF amplifier 116, and a demodulator 120.
The antenna 104 receives a radio wave sent from a transmitter (not shown). The RF-BPF 106 extracts a signal component within a certain frequency band from the received signal provided by the antenna 104. The RF amplifier 108 amplifies an output of the RF-BPF 106. The mixer 112 mixes an output of the RF amplifier 108 with a local signal generated by the local oscillator 110 to convert the output of the RF amplifier 108 into an intermediate frequency (IF) signal. The IF-BPF 114 eliminates redundant signal components from an output of the mixer 112. The IF amplifier 116 amplifies an output of the IF-BPF 114, and provides it to the demodulator 120.
FIG. 16A is a block diagram showing an arrangement of the demodulator 120 that performs a quadrature demodulation, which is one of the common demodulation schemes. The quadrature demodulator includes a phase shifter 122, a multiplier 124, a low-pass filter (LPF) 126, and a comparator 128. The phase shifter 122 shifts a phase of an IF signal to produce a phase-shifted signal. The multiplier 124 mixes the IF signal to be demodulated with the phase-shifted signal. The LPF 126 smoothes the mixed signal, and the comparator 128 discriminates a level of the smoothed signal to digitalize the smoothed signal, so that the quadrature demodulator produces a demodulated digital signal.
If a center frequency of an FSK-modulated signal is “F0” and a signal frequency of an input signal is “F”, the phase shifter 122 makes a phase rotation by 90 degrees (π/2 rad) to the input signal when F is equal to F0. It also makes the phase rotation by a certain degree that is smaller than 90 degrees when F is lower than F0, and makes the phase rotation by a certain degree that is greater than 90 degrees when F is higher than F0.
The multiplier 124 produces an output that is expressed by the following equation (21) when F is equal to F0 (phase rotation: π/2), expressed by equation (22) when F is lower than F0 (phase rotation: π/2−α), or expressed by equation (23) if F is higher than F0 (phase rotation: π/2+α), where 0<α<π/2.sin(θ)×sin(θ+π/2)={sin(2θ)}/2  (21)sin(θ)×sin(θ+π/2−α)={sin(2θ−α)+sin(α)}/2  (22)sin(θ)×sin(θ+π/2+α)={sin(2θ+α)−sin(α)}/2  (23)
By smoothing the output of the multiplier 124, a direct current (DC) component ±sin(α) appearing in the second term of the right side of the equations (22) and (23) is extracted. The DC component is positive or negative when F is lower than F0 or F is higher than F0, respectively. The comparator 128 judges the extracted signal level, thereby decoding the transmitted digital signal.
Besides the quadrature demodulator, a digital-type FSK demodulator uses a one-shot multivibrator as shown in FIG. 16B. The FSK demodulator includes a one-shot multivibrator 134, an LPF 136, and a comparator 138. As shown in FIG. 16C, the one-shot multivibrator 134 produces, from an input V1, a pulse signal V2 that has a 50% duty cycle when F is equal to F0, a duty cycle smaller than 50% when F is lower than F0, or a duty cycle greater than 50% when F is higher than F0. The LPF 136 smoothes the output V2 of the one-shot multivibrator 134. The comparator 138 compares the smoothed signal V3 from the one-shot multivibrator 134 with a threshold signal level V4 that corresponds to a 50% duty cycle, thereby decoding the transmitted digital signal.
The quadrature demodulator uses the phase shifter 122, the multiplier 124, and the LPF 126, while the FSK demodulator that has the one-shot multivibrator uses the LPF 136. Both of the demodulators include analog circuits having coils and capacitors, which are difficult to be integrated in an LSI. This prevents the demodulators from reducing in size and costs.
Another demodulator is described in JP-A-H10-173715. The demodulator has a counter that operates at a system clock frequency higher than a frequency of an input signal, and evaluates a count value at a certain timing to discriminate a phase and a frequency of the input signal. Since the demodulator includes a counter, a register, and a logic circuit, instead of using the analog circuits, the demodulator can be integrated in the LSI. Therefore, size reduction and cost reduction can be accomplished.
However, the demodulator requires the system clock that has a higher frequency than the input signal. On this account, it cannot be inputted with the input signal directly, and requires a circuit arrangement, such as a local oscillator, a mixer, an IF-BPF, and an IF amplifier, for converting the input signal into an IF signal as shown in FIG. 15. Although the demodulator itself can be made compact because it does not require the analog circuit, the receiver that includes the demodulator requires analog circuits, which has coils, capacitors, surface acoustic wave (SAW) elements, for an IF conversion as preceding stages. Therefore, this prevents the receiver from reducing in size and costs.